The present invention relates to fluidized beds and, more particularly, to a coal feed system for a fluidized bed combustor.
Fluidized beds have long been recognized as an attractive and effective means of generating heat. In these arrangements, air is passed through a bed of particulate material which normally consists of a mixture of inert material and a particulate fossil fuel such as coal, to fluidize the bed and to promote the combustion of the fuel. When the heat produced by the fluidized bed is utilized to convert water to steam, such as in a steam generator (boiler), the fluidized bed system offers an attractive combination of high heat release, improved heat transfer to surfaces within the bed and compact boiler size.
In both incinerator and boiler type arrangements, the particulate fuel material must be continuously, or at least periodically, distributed into the bed to replenish the spent material expended in the combustion process. Many conventional systems utilize a feeder for distributing the particulate fuel material from a position above the upper surface of the bed where it falls by gravity onto the latter surface. However, since a great majority of the commercial coal that is available is of a relatively large particle size range which may extend from very small particles to and including particles that will pass through a two-inch screen, this ejection of the particulate fuel material onto the upper surface of the bed presents problems. In particular, if the particulate fuel material is relatively small, the combination of the upwardly rising combustion gases and air passing through the bed will blow the material out the top of the combustor before it is burned.
It also has been suggested to provide in-bed feeding systems in which the particulate fuel material is introduced directly into the bed from a point below the upper surface of the bed. However, these in-bed systems present additional problems since the lateral transfer or dispersion of the materials through the bed is relatively poor requiring a multiplicity of feed points to prevent hot spots or over cool spots which increases the cost of the installation.